The advantage over DNSBL is that for DNSBL you first need to register the
offending
site with DNSBL. The callback is instantaneous. And the call back function
relies
on circumstances that a spammer is not likely to control, such as port
forwarding
on a router, and allowing connections from outside to port 25. Apart from the
spamer also
delivering a functional MTA, I think this is rare - this is
relying on missing features of the spamming software ala greylisting.
But as Martijn says:
I think it's far more common than you think that a sending MTA isn't
listening on port 25.
I believe it's common for companies with large mail volume to separate
the inbound and outbound mail to different servers. A result of that
may be that their outbound servers don't listen on port 25.
That said: have you done a feasibility study on this? Have you tried
putting into your inbound SMTP server, recorded the results, and
analyzed them to determine the effectiveness and false-positive rate?
Barry
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